External potential gradient control for high voltage cable terminator or bushing



1953 J. H. NICHOLAS- 2,

EXTERNAL POTENTIAL GRADIENT CONTROL FOR HIGH VOLTAGE CABLE TERMINATOR 0RBUSHING Filed June 20, 1955 jg. 1 I

25 51672: James ell icholaa United States Patent O EXTERNAL POTENTIALGRADIENT CONTROL FOR HIGH VOLTAGE CABLE TERMINATOR OR BUSHING James H.Nicholas, Chicago, Ill., assignor to G & W

Electric Specialty Company, Chicago, 111., a corporation of IllinoisApplication June 20, 1955, Serial No. 516,427

8 Claims. (Cl. 174-140) This invention relates to high voltage electricpotheads such as are used for terminating the end of an insulatedconductor, particularly a cable conductor, or for terminating aconductor constituting one terminal of a high voltage transformer orcircuit breaker. The present invention is particularly useful inconnection with potheads or cable terminations operating at very highvoltages, for example, 345 kilovolts.

In the fabrication of potheads or terminators, the sheath and screeningsurrounding the cable insulation is removed from an appropriate lengthof the cable end leaving the cable insulation exposed. The exposed cableinsulation is suitably prepared by application of stress control conesor the like in the vicinity of the cut back ends of the cable sheath andscreening. The cable end is then surrounded by a suitable insulatorbushing having the usual axially spaced insulating skirts. The lower endof the insulator bushing is mounted on a grounded metal support. At theupper end of the insulator bushing is a line terminal to which isconnected the end of the cable conductor and an external transmissionline.

The outer insulator bushing and internal insulation must withstand theaxial voltage gradient existing between the cable conductor and ground.To minimize the length of the insulator bushing required to withstandthis potential difference, the potential gradient of the insulatorbushing should be uniform along the length thereof.

It is one of the objects of this invention to provide means forimproving the voltage distribution, in an axial direction, internallyand externally of the pothead, utilizing an external voltage controlunit, preferably a condenser stack of a type used in high voltagetransmission systems, mounted adjacent to the terminator.

In accordance with a preferred aspect of the invention, the insulatorskirts are omitted from points along the length of the insulator bushingand the insulator surface is coated with a conductive glaze at thesepoints. A separate stack of series connected capacitor units is mountedadjacent to the terminator, and the capacitor stack is connected betweenthe aforementioned cable conductor terminal and ground. Connections arealso made between intermediate terminals of the capacitor stack and theglazed areas on the insulator bushing to control the voltagedistribution axially along the bushing and thus also controlling thevoltage distribution axially within the bushing. Due to the flow ofprogressively increasing capacitive currents proceeding toward thegrounded end of the terminator, the capacitance of the capacitors in thestack are made of progressively increasing value from the high voltageend of the stack. In this maner, the voltage gradient lengthwise of theinsulator bushing is uniform.

Other objects, advantages and features of the invention will becomeapparent upon making reference to the specification to follow, taken inconjunction with the drawings wherein:

Fig. 1 is an elevational view of a terminator and capacitor stackforming a preferred embodiment of the invention; and

2,820,086 Patented Jan. 14, 1958 Fig. 2 is a fragmentary enlargedsection through part of the outer insulator bushing of the terminator.

Reference should now be made to the drawings where similar referencenumerals indicate similar elements throughout.

In Fig. 1 is shown a terminator structure 1 including a slightly taperedouter insulator bushing 3 which encloses the end portion of a cable 2entering the terminator from the bottom thereof. The cable end may beprepared in any suitable manner Well known in the art. One method is toremove a portion of the cable shielding braid and shielding to leaveexposed cable covering insulation 2. The covering insulation may besuitably prepared in a manner well known in the art, as by the additionof a stress control cone adjacent to the cut-back end of the shieldingbraid. The latter is usually located near the bottom of the terminator,as shown in co-pending application Serial No. 219,294, filed April 4,1951, on a High Voltage Electric Terminator. The cable conductor isconnected to a terminal located within the insulator at the top of theterminator. The cable terminal connects with an overhead conductor whichenters the top of the terminator through a line terminal 4. The lineterminal 4 is connected to a hollow metal shield piece 5 which ismounted above a metal adaptor ring 6 secured to the upper neck 7 of theouter insulator bushing. The adaptor ring 6 has an internally threadedboss 8 for receiving the threaded end of a metal connector rod 10.

The outer insulator bushing 3 is supported on a grounded metal support12 having a metal mounting flange 13, as by screws 14, and a metaladaptor ring 15 at the bottom of a neck 16 of the bushing 3.

The terminator insulator bushing 3 of the invention is preferably madeof porcelain and has the usual insulating skirts 19. At locations 21,22, 23 and 24 uniformly spaced between ground and the line potential endof the bushing there are pairs of oppositely extending peripheralprojections 2627 which provide annular insulator surfaces 28 (Fig. 2)having a C-shaped profile. These surfaces each have a generallycylindrical or slightly tapered intermediate exposed insulator bodysurface 29 which join oppositely curving insulator body surfaces 30-30recessed within the insulator bushing. The insulator bushing surfaces 28are coated with a conductive glaze 32 following the G-shaped profile ofthese surfaces.

Split metal rings 31 surround each of the conductive glazed areas 21through 24 of the insulator bushing and are tightly secured thereto.Each ring has an internally threaded boss 31 which receives the end ofan associated connector rod 10'.

Means are provided for applying or controlling the voltage at the points21 through 24 so that the voltage gradient along the insulator bushingis uniform. As is apparent from the description above, the operatingvoltage of the cable system with which the terminator is associatedexists between the upper adaptor ring 6 and the bottom adaptor ring 15of the insulator bushing 3. A proper voltage gradient along theinsulator bushing above described is obtained by fixing the voltage atthe points 21, 22, 23 and 24, respectively, 20%, 40%, 60% and of theoperating voltage of the cable system with which the terminator isassociated.

A condenser assembly 11 controls the potential gradient along theinsulator bushing 3.

The condenser assembly is supported on a hollow base 33 which in turn ismounted upon a support wall 17 which also supports the terminator 1.

The condenser assembly 11 comprises a stack of seriesconnected condenserelements 34, 35, 36, 37 and 38.

3 the stacking of one element on top of the other. To this end, eachelement has mounting flanges 40. The mountmg flanges of adjacentcondenser elements are bolted together by bolts 40'. The flanges alsoform terminals for the associated condenser elements, and the condenserelements are each thereby connected in series with the adacent capacitorelement.

The opposed mounting flanges 40-40 form bosses 41 which receive the endsof the horizontally extending connector rods ltl' extending from thesplit rings 31 of the termmator.

The uppermost terminal of the uppermost capacitor element 38 of thestack is electrically connected to and covered by a hollow, generallyspherical electrostatic sh eld 42. The connector rod 19 electricallyjoins the shleld 42 with the line terminal 4 of the terminator. Thelower terminal of the bottom condenser element 34 of the stack isconnected to the base 33 of the condenser assembly which in turn isgrounded through the metal support l7. The condenser stack assembly istherefore connected in series between points of the terminator having,respectlvely, line and ground potential.

It can be shown that the currents flowing in the condenser elements 34through 38 increase progressively from the top to the bottom of thecondenser stack. Therefore, to equalize the voltage drop across thecondenser elements the values of the elements are graded in proportionto the currents flowing through them, so that the same voltage dropappears across each of the elements. In this manner, voltage at points21 through 24 of a terminator insulator bushing is held at respectively20, 4t), 69 and 80 percent of the line voltage, respectively.

in compliance with the requirements of the patent statutes l have hereshown and described a preferred embodiment of my invention. It is,however, to be understand that the invention is not limited to theprecise construction here shown, the same being merely illustrative ofthe princlples of the invention. What I consider new and desire tosecure by Letters Patent is:

1. In combination, a terminator for a high voltage cable, saidterminator including an outer insulator into which the cable or cableconductor is to extend, a line terminal at one end of the insulator, anda grounded member at the opposite end of the insulator; and an impedanceunit mounted separately of said terminator to one side of saidterminator and being electrically con nected between said groundedmember and said line ter minal, said impedance unit having a number oftap-oft points therealong, and respective connector means connectingaxially spaced portions on said outer insulator to the taped points ofsaid impedance unit which fix the potentials of said axially spacedportions on said insulator, the sum of the axial lengths of the portionsof the outer insulator that are connected to the impedance unit beingless than the sum of the axial lengths of the insulator between saidportions.

2. In combination, a terminator for a high voltage cable, saidterminator including an outer insulator into which the cable or cableconductor is to extend, a line terminal at one end of the insulator, anda grounded member on which the insulator is mounted at a location spacedfrom the line terminal end thereof; and an impedance unit mountedseparately of said terminator and being electrically connected betweensaid grounded member and said line terminal, said impedance unit havinga number of tap-off points therealong, and respective connector meansconnecting axially spaced portions on said outer insulator to thetap-off points of said impedance unit which fix the potentials of saidaxially spaced portions on said'insulator at values providing apredetermined potential gradient along said insulator, the sum of theaxial lengths of the portions of the outer insulator that are connectedto the impedance unit being less than the sum of the axial lengths ofthe insulator'be'twe'e'n said portions.

3. In combination, a terminator for a high voltage cable having an outerinsulator into which the cable or cable conductor is to extend, a lineterminal at one end of the insulator, a grounded member surrounding theinsulator and spaced from the line terminal end thereof, and a number ofseparate conductive areas contiguous to and surrounding said outerinsulator at axially spaced locations between the line terminal end andthe grounded member, the sum of the axial lengths of the conductiveareas being less than the sum of the axial lengths of the insulatorbetween said conductive areas; and an impedance unit mounted to one sideof said insulator and electrically connected between ground and the lineterminal, said impedance unit having a number of tap-off pointstherealong, and respective connector means connecting the conductiveareas of said insulator to the tap-oil points of said impedance unitwhich fix the potentials of said conductive areas at values providing auniform potential gradient along said insulator.

4. In combination, a terminator for a high voltage cable including anouter insulator having an end into which the cable conductor extends, aline terminal at the other end of the insulator which terminal isconnected to the cable conductor, a grounded member at saidfirstmentioned end or" the outer insulator, and a number of separateconductive areas contiguous to and surrounding said outer insulator atequally and axially spaced locations between the line terminal end andthe grounded member, the sum of the axial lengths of the conductiveareas being less than the sum of the axial lengths of the insulatorbetween said conductive areas; and an impedance assembly mountedseparately and to one side of said terminator, said impedance assemblycomprising a number of series connected impedance elements ofprogressively increasing value, means connecting the high impedance endof the impedance assembly to said line terminal, means connecting thelow impedance end of said impedance assembly to said grounded member,and means connecting the respective junctures of said impedance elementsto corresponding conductive areas of said insulator.

5. In combination, a high voltage outer insulator having a line terminalat one end and a grounded member surrounding the insulator and spacedfrom the line terminal end, and a number of separate conductive areascontiguous to and surrounding said outer insulator at axially spacedlocations between the line terminal end and the grounded member, the sumof the axial lengths of the conductive areas being less than the sum ofthe axial lengths of the insulator between said conductive area; and animpedance assembly mounted separately and to one side of said insulatorand electrically connected between said line terminal and groundedmember, said impedance assembly comprising a number of series connectedimpedance elements, and means connecting the respective junctures ofsaid impedance elements to corresponding conductive areas of saidinsulator, the relative values of said impedance elements being suchthat the voltage drops therein are proportional to the spacing betweenthe spaced conductive areas to which they are connected so that theaxial voltage gradient along the outer insulator is uniform.

6. In combination, a terminator for a high voltage cable comprising anouter insulator having an end into which the cable extends, a lineterminal at the other end of said outer insulator, a grounded member atthe first-mentioned end of the outer insulator, and said outer insulatorhaving a number of axially spaced insulating skirts which areinterrupted at a number of axially spaced points by pairs of oppositelyextending peripheral projections providing annular insulating surfaceshaving 0- shaped profiles, each of said annular insulator surfaces beingcoated with a conductive glaze; and an impedance unit connected betweensaid line terminal and said ground member, said impedance unit having anumber of tap-off points therealong, and means connecting said glazedareas of said insulator to tap-oil points of said impedance unit asaopsewhich fix the potential of said glazed areas at values providing auniform potential gradient along said insulator.

7. In combination, a terminator for a high voltage cable comprising anouter insulator having an end into which the cable is to extend, a lineterminal at the other end of said insulator, a grounded member at saidfirstmentioned end of the insulator, said insulator having a number ofaxially spaced pairs of oppositely extending peripheral projectionsproviding annular insulator surfaces, respective conductive bandssurrounding said an nular surfaces, the sum of the axial lengths of therespective conductive bands being less than the sum of the axial lengthsof the insulation between the bands, and an impedance unit connectedbetween said line terminal and grounded member and having tap-off pointsconnected to said conductive bands on said outer insulator to controlthe potential of the latter to provide a uniform potential gradientalong the insulator.

8. In combination, a terminator for a high voltage cable including anouter insulator having an end into which the cable extends, a lineterminal at the other end of the insulator which terminal is connectedto the inner cable conductor of the cable, a grounded member at saidfirst-mentioned end of the outer insulator, and a number of separateconductive areas contiguous to and surrounding said outer insulator ataxially spaced locations between the ends thereof, the sum of the axiallengths of the conductive areas being less than the sum of the distancesbetween the conductive areas; and an impedance assembly mountedseparately and to one side of said terminator and electrically connectedbetween said line terminal and ground, said impedance assemblycomprising a number of series connected impedance elements, and meansconnecting the juncture of said impedance elements to correspondingconductive areas of said insulator, the relative values of saidimpedance elements being graded from a maximum at one end to a minimumat the opposite end.

References Cited in the file of this patent UNITED STATES PATENTS1,869,168 Proctor et al. July 26, 1932 20 2,068,624 Atkinson Jan. 19,1937 2,423,596 Hollingsworth July 8, 1947 FOREIGN PATENTS 352,635 GreatBritain July 16, 1931

